Metered-dose inhalers include pressurised metered-dose inhalers (of both manually operable and breath-actuated types) and dry-powder inhalers. Such metered-dose inhalers typically comprise a medicament-containing vessel and an actuator body having a drug delivery outlet. The medicament-containing vessel may be a pressurised canister containing a mixture of active drug and propellant. Such canisters are usually formed from a deep-drawn aluminium cup having a crimped ferrule which carries a metering valve assembly. The metering valve assembly is provided with a protruding valve stem which, in use, is inserted as a tight push fit into a so-called “stem block” in the body.
To actuate the conventional manually operable inhaler, the user applies a compressive force to the closed end of the canister. The internal components of the metering valve assembly are spring loaded so that a compressive force of about 15 to 30 N is required to activate the device. In response to this compressive force, the canister moves axially with respect to the valve stem by an amount varying from about 2 to 4 mm. This degree of axial movement is sufficient to actuate the metering valve and cause a metered quantity of the drug and propellant to be expelled through the valve stem. This is then released into the mouthpiece via a nozzle in the stem block. A user inhaling through the drug delivery outlet of the device at this point will thus receive a dose of the drug.
Metered-dose inhalers as described above administer an accurate dose of medicament whenever required, which is particularly useful for users whose respiratory difficulties manifest themselves suddenly. Such has been the success of these devices that they are now used throughout the world.
A more recent development is the so-called breath-actuated metered-dose inhaler which delivers a dose of drug through a mouthpiece in response to inhalation by the user. This type of arrangement is particularly convenient in circumstances where the co-ordination between user inhalation and manual depression of the aerosol canister is imperfect. For example, children sometimes lack the necessary co-ordination to achieve effective self-administration and, at times of respiratory distress, adult users may also experience poor co-ordination.
One of the drawbacks of self-administration from an inhaler, whether manually operated or breath-actuated, is that users often experience difficulty in determining when the charge in the medicament-containing vessel has nearly run out, since the contents of the medicament reservoir are typically invisible to the user. With aerosol canisters, part of the reason for this difficulty is that a surplus of propellant may remain in the canister even though the drug supply is nearly exhausted. Alternatively, the near-exhausted state may result in a surplus of drug in relation to propellant. Thus, the illusion is created that the inhaler is still capable of providing useful doses of medicament simply because the canister contains liquid. This is potentially hazardous for the user since dosing becomes unreliable and because few users routinely carry a back-up device. Many users have several different inhalers for the treatment of a variety of conditions. Others keep inhalers at a number of different locations such as at school, home, work etc. In these circumstances it is particularly difficult for the user to keep track of the amount of usage extracted from each individual inhaler apparatus.
WO 98/28033 discloses a dose counter suitable for use with the above-described metered-dose inhalers. The dose counter enables users to assess how many doses remain in the obscured canister. Such a counter can provide a warning when the inhaler nears exhaustion so that appropriate measures can be taken to avoid running out of medication. Moreover, since the dose counter has a counting resolution of one dose, it can be used for compliance monitoring, either under hospital supervision or by parents and teachers assessing compliance by children in their care. Furthermore, there are regulatory requirements for metered-dose inhalers to have a dose counter in a number of countries.
FIGS. 1 to 3 reproduced herein from WO 98/28033 show the lower portion of a metered-dose inhaler. The inhaler comprises a body 2 having a drug delivery outlet 4. An aerosol canister 6 extends into the lower portion of the body 2. The aerosol canister 6 is formed from a deep-drawn aluminium cup 8 to which a ferrule 10 is attached by crimping.
The lid 10 carries a metering-valve assembly having a protruding valve stem 12, the end of which is received as a tight push fit in a stem block 14 of the body 2. Stem block 14 has a nozzle 16 communicating with the drug delivery outlet 4 so that, upon actuation of the metering-valve assembly, a charge of the drug is emitted through the nozzle 16 into the drug delivery outlet 4. Actuation of the metering-valve assembly is effected by causing downward movement of the aerosol canister 6 relative to the body 2. This may be achieved through manual pressure exerted by the user against the upturned base (not shown) of the aerosol canister 6 or by automatic depression of the aerosol canister 6 in response to user inhalation in inhalers of the breath-actuated type. The mechanism of actuation does not form part of WO 98/28033 or the present invention and will not be described in further detail. A user inhaling through the drug delivery outlet 4 when the aerosol canister 6 is depressed will receive a metered dose of the drug.
With reference to the Figures, a counter mechanism 18 includes an actuator shaft 20 moulded from a plastics material, such as nylon, the actuator shaft 20 having a boss 22 integrally formed at its base. The underside of boss 22 is formed with a blind hole which receives a compression spring 24 mounted on an upstanding spigot 26 formed on a lower element of the counter chassis.
A driver 28 for driving a rotary gear in the form of a ratchet-toothed wheel 30 is integrally moulded with boss 22 of the actuator shaft 20 and comprises a transverse hook element mounted between two arms (only one of which is visible in FIG. 2), the bases of which are conjoined to the boss 22. The transverse hook is dimensioned and oriented to engage with ratchet teeth 32 formed around the periphery of the ratchet-toothed wheel 30 to rotate it in a forward direction.
The ratchet-toothed wheel 30 is integrally moulded with a first hollow axle 34 which is rotatably supported on a first spindle 36 that projects transversely from a chassis sub-element 38. Chassis sub-element 38 also has a second spindle 40 projecting transversely therefrom on which a second hollow axle 42 is rotatably supported. A flexible tape 44 is wound around the second hollow axle 42 which serves as a supply spool and passes to the first hollow axle 34 which serves as a take-up spool (stock bobbin). A guide plate 46 forming part of the chassis sub-element 38 helps to guide the tape 44 in a smooth passage from the supply spool to the take-up spool. The surface of the tape 44 is marked with a progression of descending numbers which denote the number of doses remaining in the aerosol canister. Typically, the starting count is 200 and successive markings on the tape decrease by one. Markings on the tape actually decrease by two. The spacing between successive markings is coincident with the indexing motion of the ratchet-toothed wheel 30 so that a new number appears in a window 48 provided in the body 2 for each successive actuation.
The ratchet-toothed wheel 30 and integrally formed first hollow axle 34 are restrained from reverse rotation by a wrap-spring clutch 50 surrounding the hollow axle 34 at the end thereof remote from ratchet-toothed wheel 30. One end (not shown) of the wrap-spring clutch 50 is braced against the counter chassis. The windings of the wrap-spring clutch 50 are oriented such that rotation of the first hollow axle 34 in a forward sense is not resisted by the spring coils. However, reverse rotation of the hollow axle 34 acts so as to tighten the spring coils around it, thereby causing the first hollow axle 34 to be gripped by the internal surface of the wrap-spring clutch 50 and hence restraint from reverse rotation.
FIG. 3 shows a more detailed view of the principal elements of the dose counter 18. It will be seen that the driver 28 comprises the transverse hook 52 mounted between a pair of arms 54, 56 which are joined at their bases by a web. The web is connected to the boss 22 of the actuator shaft 20. A combined actuator and driver assembly may be integrally formed, such as from a plastics material, e.g. as nylon.
In use of the dose counter 18, depression of the canister 6 causes the ferrule 10 to engage with the actuator shaft 20, which actuator shaft 20 moves downwards against the compression spring 24. The transverse hook 52, in turn, engages with the ratchet teeth 32 of the ratchet-toothed wheel 30 which is mounted on the hollow axle 34 serving as the take-up spool for the flexible tape display 44. At the end of the hollow axle 34 remote from the ratchet-toothed wheel 30 is the clutch 50 which serves to restrain the axle 34 against reverse rotation and hence prevents reverse travel of the counter tape 44.
A control surface 58 is depicted in FIG. 3 as a see-through element so that the workings of the dose counter may be more clearly seen. The control surface 58 extends parallel to the direction of travel of the actuator shaft 20 and is located adjacent the ratchet-toothed wheel 30 at a position which marks a chordal projection across one of the wheel faces. One of the support arms 56 of the driver 28 is in sliding contact with control surface 58. This sliding contact serves to inhibit the natural tendency of the driver 28 to flex radially inwardly towards the axis of rotation of the ratchet-toothed wheel 30. By preventing such radially inward flexure, the control surface 58 restricts the engagement and disengagement of the drive 28 with the ratchet-toothed wheel 30 so that the distance by which the ratchet-toothed wheel 30 rotates is limited to one tooth pitch. This condition is observed regardless of the extent of linear travel, or stroke, of the actuator shaft 20.
FIG. 4 shows a schematic view of an alternative arrangement for the ratchet-toothed wheel and driver used in the dose counter 18 described in WO 98/28033. The alternative arrangement uses a reciprocating driver 28 acting in a pushing sense to rotate a ratchet-toothed wheel 30 in the direction shown by the arrows 31. A fixed pawl 60 acts to prevent reverse rotation of the ratchet-toothed wheel 30 by engagement against the trailing edge 62 of a ratchet tooth 32. However, on forward rotation of the ratchet-toothed wheel 30 in the sense of arrows 31, the fixed pawl 60 is capable of radially outward deformation, urged by the leading edge 63 of a ratchet-tooth 32.
In this arrangement, if the ratchet-toothed wheel 30 is rotated by more than a single tooth pitch but by less than two tooth pitches for each reciprocating movement of the driver 28, there is a degree of reverse rotation until the pawl 60 becomes engaged by the trailing edge 62 (as opposed to the leading edge 63) of a ratchet tooth 32. Thus, the rotation of the ratchet-toothed wheel 30 may be described as “stepped”.
The components of metered-dose inhalers are manufactured to a high technical specification. However, inevitable variations in the tolerances of the components can, in some circumstances, lead to failure of the dose counter of the type disclosed in WO 98/28033. In a known failure mode, the reciprocating stroke of the canister is insufficient to fully increment the dose counter. This may lead to undercounting, particularly where rotation of the ratchet-toothed wheel is stepped, as illustrated in FIG. 4.
Another problem relates particularly to manually operated metered-dose inhalers. In these types of inhaler, the user cannot be relied upon to repeatably actuate the inhaler with a full reciprocating stroke of the canister. Instead, the user may on some occasions release the canister immediately after the “fire point” of the metering valve, that is to say the point in the stroke at which the medicament is dispensed. This reduced stroke of the canister available for incrementing the dose counter may exacerbate the problem described above.
There is a requirement in the art, therefore, for a dose counter with a reduced failure rate. There is a particular requirement for such a dose counter which can be manufactured efficiently and incorporated into known metered-dose inhalers.